Photo-resist is often used in semiconductor manufacturing in order to fabricate structured layers such as redistribution layers on a semiconductor wafer. The photo-resist may be applied to the semiconductor wafer in a liquid form known as a wet resist. The wafer then may be spun so that the layer of photo-resist on the surface of the wafer has a uniform thickness. Alternatively, the photo-resist may be applied to the semiconductor wafer in a dry form known as dry film resist. The dry film resist is often provided in the form of a film, which may be delivered on a roll. Typically, the dry film resist is supplied in a form of a laminated film with a film of photo-resist sandwiched between a cover film and a carrier film. The dry film resist may be applied to a substrate, such as a printed circuit board, or to a wafer by lamination techniques.
The wet resist may be associated with some peculiarities in some circumstances. For example, if a thick layer of resist, such as more than fifty micrometers, is desired as a template for electrochemical deposition of alloys that may later serve as a bumps in a wafer bumping process. The wet resist having a low viscosity can be used to produce the thick layer. However, the uniformity of the thickness of the layer produced in this way can vary to a degree. In contrast, the dry film resist can be produced in relatively large thicknesses of fifty um or more with a very high accuracy.
Another aspect of the wet resist occurs for coating a wafer surface that has certain topography, such as cavities that are found often in micro-electromechanical system (MEMS) applications. With the wet resist, it may not be possible to achieve a homogenous coverage of the surface due to surface tension at sharp corners of the topography. However, the dry films have a certain internal stiffness, which may allow them to cover evenly such topography.
U.S. Pat. No. 6,623,912 discusses a method in which the dry film resist is unwound from a storage roll to a take-up roll. Two rollers and a heat and pressure roller are positioned so that the film contacts a semiconductor wafer over a heat stage. After the dry resist film is laminated on the wafer by applying the heat and pressure roller, a cutting laser is used to separate the portion of dry resist film now attached to the wafer from the bulk of the dry resist dry film resist on the roll.